I've been a member of the Authors Guild for ten years, and I recommend it, even though I can't benefit from the health insurance. Through the AG, I maintain my website, at nominal cost (tax deductible, of course), using the free web-building tools, a huge help for a computer idiot like me. The booklet the AG puts out on contracts is a must-have; it explains what the clauses mean to the author, what you should ask for, and what you shouldn't accept. The AG's legal staff will vet a contract for you for free or answer a question relating to it; I once had a question about foreign rights that was resolved quickly and clearly.
Though there's a lot of fluff in the quarterly newsletter, it, too, can be useful, particularly concerning legal issues like copyright or libel. Twice a year or so, their round-table discussions (whose transcripts appear in the newsletter) offer worthwhile information on industry trends.
Finally, the Authors Guild takes stances I approve of, opposing censorship and standing up for authors' rights, often filing friends-of-the-court briefs on issues like the Mickey Mouse copyright law, or taking a more active position, as with the suit against Google. Most recently, the AG was mobilizing opinion against the dead celebrities bill before the New York State legislature, brought up on AW a few days ago.
I wouldn't mention membership in the AG as a hook for an agent or editor. The only thing it proves is that you've been published, and your query will already say that.
Ninety bucks a year is real money, even though it's tax deductible. But I think I've gotten good value for it.
